In wireless communication systems, antenna arrays are used at devices on one or both ends of a communication link to suppress multipath fading and interference, and to increase received signal power at a receiving device. In certain wireless communications systems like WiMAX™, a base station (BS) device and each of a plurality of mobile station (MS) devices are equipped with a plurality of antennas. The BS device can beamform signals to a particular MS device by weighting transmit signals in phase and magnitude for transmission via its plurality of antennas to the particular MS device. In so doing, the signals received at the particular destination device can be coherently combined to provide higher received signal power and reduced interference from other devices. The BS device uses knowledge about the over-the-air channel between it and the particular destination device to choose values for the antenna weights applied to the transmit signals.
However, certain information, such as preamble data, is intended for reception by multiple (e.g., all) destination devices within the coverage area of BS device. There are other situations where a message is to be transmitted to a particular device, but the location or spatial signature of that device is unknown. If a message is intended for a particular device whose location or spatial signature is unknown, or if a message is intended for multiple devices within a coverage area, then the BS device broadcasts the message within the coverage area using a radiation pattern that covers the entire coverage area or a desired portion of the coverage area as the case may be. Generally, the BS cannot beamform a broadcast message to multiple intended destination devices even if their locations are known, nor can the BS beamform a message to a destination device whose location or spatial signature is not known.
There are several methods to broadcast messages with an antenna array. A first method is to send the broadcast message through a single antenna. The radiation pattern of a single antenna spans the entire coverage area (or a desired portion of the coverage area) and therefore the broadcast message will be sent ‘omni-directionally’, i.e., the message will be transmitted according to the radiation pattern of the single antenna. The drawback of this approach is that the broadcast message will have significantly less coverage along the radius of the radiation pattern because a single antenna does not have beamforming gain or power combining gain. A second approach is to send the broadcast message using a synthesized beam pattern that spans the whole coverage area or a desired portion of the coverage area. However, beam pattern synthesis has stringent requirements on the antenna array configuration and calibration, and is not always feasible.
Accordingly, a technique is needed for transmission of broadcast messages using multiple antennas that can achieve gain at the destination devices without imposing constraints on antenna configuration and without for the need for antenna calibration.